This invention relates to the field of compact, radio frequency (RF) transponders of the type known to be useful in systems for security and information storage, access control, entry validation and identification, and in other comparable systems. Such a system requires an interrogator circuit built into a road vehicle or building, for example, and a remote transponder which incorporates transmitting and receiving circuits in a compact case that may be carried by a person in a key, a key fob, a badge, a tag or in any similar miniaturized housing. More particularly this invention relates to a transponder in a road vehicle or automotive remote keyless entry and immobilization system which is functional over an increased range in active and passive modes of operation. This invention further relates to a transponder which utilizes a secure challenge-response encryption technique to provide greater security for the user.
Compact passive low frequency transponders, using a frequency of 134.2 kilohertz (134.2 kHz), for example, for passive entry and immobilizer functions and radio frequency remote control transmitters, using a frequency of 433 megahertz (433 MHz), for example, for use in remote keyless entry and security systems for automobiles are generally known. These systems allow access to the automobile without the use of battery power, if the transponder is used in close proximity to the interrogator, and allow the operator to transmit commands such as locking and unlocking doors, hood and trunk, controlling vehicle lighting and ignition, and arming and disarming the anti-theft security system to the vehicle over greater distances. The transponders used may employ an interrogator-responder arrangement with an EEPROM data storage device and a small capacitor that serves as an energy accumulator, charged by the energy provided by the radio frequency interrogation, to provide power for the transponder. The transponder is, thus, sufficiently small to supplement or replace a conventional vehicle door and ignition key. Such a transponder is disclosed in Schuermann et al., U.S. Pat. No. 5,053,774, which is incorporated herein by reference.
However, the transponder systems in current use generally have a limited operating range. Current remote control transponder systems require battery power for proper operation and are not functional, in a passive mode, that is, when operated without a battery.